The present invention relates to a method of mass spectrometry and a mass spectrometer.
Historically, Multiple Reaction Monitoring (“MRM”) experiments to select and quantitate specific ions in a complex sample have been carried out using tandem quadrupole instruments wherein a first analytical quadrupole mass filter is arranged to select or isolate specific parent or precursor ions of interest. A fragmentation device is located downstream of the first analytical quadrupole mass filter and is arranged to fragment the parent or precursor ions of interest to form fragment ions. A second analytical quadrupole mass analyser located downstream of the fragmentation device is then arranged to mass analyse the characteristic fragment ions.
Desired parent or precursor ion to fragment ion transitions are determined via a method development stage wherein the transitions are arranged as a function of elution time from a chromatographic device such as a Liquid Chromatography (“LC”), Gas Chromatography (“GC”) or Capillary Electrophoresis (“CE”) device. In some applications multiple transitions per parent or precursor ion may be arranged in order to give an added level of confidence that the measured component is actually the correct one.
Recently, it has become apparent that the specificity or selectivity of tandem quadrupole MRM experiments, particularly in relation to complex mixtures such as those seen in proteomics, is insufficient in some circumstances.
In order to address this problem it is known to perform high resolution MRM experiments using a mass spectrometer wherein the second analytical quadrupole mass analyser is replaced with a higher resolution mass analyser such as an Orbitrap® mass analyser or an orthogonal acceleration Time of Flight mass analyser.
FIG. 1 shows a current state of the art mass spectrometer that may be utilised to perform high resolution MRM experiments.
It is known to provide a quadrupole mass filter 2 in conjunction with an orthogonal acceleration Time of Flight mass analyser 4 as shown in FIG. 1 wherein parent or precursor ions are isolated or selected by the quadrupole rod set mass filter 2 and are then subsequently fragmented in a gas cell 3. The resulting characteristic fragment ions are then mass analysed using the high resolution orthogonal acceleration Time of Flight mass analyser 4.
Using a high resolution orthogonal acceleration Time of Flight mass analyser 4 has several advantages compared with using a resolving analytical quadrupole mass analyser.
Firstly, the higher resolution orthogonal acceleration Time of Flight mass analyser 4 reduces the likelihood of an interference effecting the quantitative measurement of the characteristic fragment ions.
Secondly, the orthogonal acceleration Time of Flight mass analyser 4 inherently has a high mass measurement accuracy of the order of 1-3 ppm RMS. This mass accuracy can be used to improve the specificity of the transition.
Thirdly, the orthogonal acceleration Time of Flight mass analyser 4, by virtue of the fact that it is a mass spectrometer as opposed to a mass filter, analyses multiple ions (characteristic fragment ions in this case) simultaneously and with a high duty cycle. The resulting full mass spectral data contains multiple characteristic fragment ions for the same parent or precursor ions again improving the specificity. Multiple isotopes are also included in the full mass spectrum again improving specificity.
Despite these benefits, current state of the art mass spectrometers similar to the arrangement shown in FIG. 1 nonetheless suffer from some certain problems.
One problem with current state of the art mass spectrometers is that they suffer from some loss in duty cycle as a result of mass analysing ions using an orthogonal acceleration Time of Flight mass analyser 4.
It is known to seek partially to compensate for this by either operating the Time of Flight mass analyser 4 in an Enhanced Duty Cycle (“EDC”) mode of operation wherein the mass range of ions analysed at any point in time is reduced or alternatively by operating the Time of Flight mass analyser 4 in a High Duty Cycle (“HDC”) mode of operation or a scanwave/zeno lens mode of operation wherein the dynamic range is reduced.
Typically, the dynamic range of the ion detection system used in conjunction with an orthogonal acceleration Time of Flight mass analyser 4 is inferior to that of a quadrupole rod set mass analyser due to the high digitisation rate requirements of an orthogonal acceleration Time of Flight mass analyser 4.
Current state of the art Time of Flight mass analysers employing Analogue to Digital Converters (“ADC”) exhibit significant improvements compared with previous mass spectrometers that used Time to Digital Convertors (“TDC”). Future developments in digitations rates and/or multiple gain stage ADCs promise further improvements.
It is known to seek to improve the dynamic range of an orthogonal acceleration Time of Flight mass analyser by means of Programmable Dynamic Range Enhancements (“pDRE”) and Automatic Gain Control (“AGC”). However, these approaches usually involve a loss in sensitivity and/or duty cycle.
State of the art instruments that employ quantitative acquisitions use the same ion or ions for quantification irrespective of the nature of the data.
It is known to calibrate a mass spectrometer by injecting a first calibration sample having a known (low) concentration of calibrant and then measuring the low intensity signal response. Second and further calibration samples having progressively higher calibrant concentrations are then sequentially injected or other introduced into the mass spectrometer and increasingly higher signal intensities or responses are observed.
At a certain point the ion detector will start to saturate and the detector response will not increase further. This gives an indication of the upper limit of quantitation and provides an upper limit to the effective dynamic range of the mass spectrometer.
It is desired to provide a mass spectrometer and a method of mass spectrometry having an improved dynamic range.